1. Field of the Invention
The present invention relates generally to cryptography and, more particularly, to quantum cryptography systems and methods.
2. Description of the Related Art
The worldwide proliferation of digital communication fuels an ongoing quest for more secure and efficient modes of conveying information. In the context of the Internet, for example, digital exchanges between geographically separated parties may pass through any number of non-secure intermediate devices and thus invite a wide variety of unwanted third party interference, including eavesdropping, tampering and impersonation. Such interference in turn drives development and use of an equally wide variety of preventative measures.
The art of cryptography represents one such measure. In cryptography-based systems, a sending party encrypts a message to produce a corresponding cipher message, or cryptogram, and transmits the cipher message to an intended recipient via a potentially non-secure communication channel. The intended recipient then decrypts the cipher message to recover the original message. Since the cipher message is, at least in theory, unintelligible to anyone other than the sender and the intended recipient, a potentially malicious third party with access to the communication channel cannot readily eavesdrop or otherwise interfere.
Quantum cryptography (i.e., cryptography using applied quantum mechanics) is one type of cryptography that has certain advantages. In single-photon quantum cryptography, for example, cipher symbols (also known as “values”) may be encoded via non-commuting observables of individual photons conveyed from a sender to an intended recipient (e.g., via an optical fiber). Because quantum mechanics guarantees that a third party cannot divide a single photon, or simultaneously discern precise values for non-commuting observables of a single photon, the sender and the intended recipient may work together to develop secure and mutually agreed upon ciphers for use in subsequent cryptographic exchanges.
Quantum cryptography based on highly attenuated laser sources (“Weak Coherent” quantum cryptography) may be vulnerable to advanced forms of eavesdropping. In particular, one attack may be termed a Photon Number Splitting (PNS) attack. This attack exploits the fact that attenuated sources are not true single-photon sources, but rather produce photons by a Poissonian process. Thus sometimes multiple photons may be emitted instead of a single photon. The attack determines which emitted pulses in fact contain multiple photons. It suppresses all other pulses, extracts one or more photons from the multi-photon pulses and then analyzes them, and then delivers the remaining photon (or its synthesized surrogates) to the intended receiver. In this manner, the eavesdropper can reliably learn the contents of the supposedly secure transmission of raw key material, and hence may be able to intercept, read, and fabricate messages. Several other such attacks have been proposed in the specialist literature, based on the emission characteristics of attenuated Weak Coherent laser sources.
To remedy these shortcomings, some research teams have proposed and occasionally built forms of quantum cryptographic systems based on the production and detection of pairs of entangled photons, such as the process of Spontaneous Parametric Downconversion. Such systems appear to overcome some weaknesses of Weak Coherent systems. However, existing systems based on entanglement do not provide any means for performing Path Length Control which is the process of continually readjusting two widely separated interferometers (one at the transmitter and the other at the receiver) to maintain an exact relationship between their lengths, even as temperatures of the interferometers rise and fall, etc. Nor do they provide any facility for generating numbered frames of entangled photons rather than a single, long, unnumbered series of such photons. In addition these systems lack convenient facilities for debugging and trouble-shooting. Consequently, a need exists for improved forms of quantum cryptography based on entanglement of photons.